The tumor necrosis factor receptors (TNFR) are a superfamily of transmembrane receptors involved in cell communication within the immune system. TNFR family members are structurally characterized by extracellular cysteine-rich domains (CRD) that form ligand-binding motifs. Generally, members of the TNFR superfamily found on B or T cells are type I transmembrane proteins that have several CRDs (Bodmer, J. L., et al., (2002) Trends Biochem Sci 27:19-26). There is, however, a sub-group of TNFR proteins expressed by B cells that are type III transmembrane proteins and contain a reduced number of CRDs: B-Cell Maturation Antigen (BCMA), Transmembrane Activator and CAML Interactor (TACI), and BLyS (BAFF) Receptor 3 (BR3) (Gross, J. A., et al., (2000) Nature 404: 995-999; Marsters, S. A., et al., (2000) Curr Biol 10:785-788; Thompson, J. S., et al., (2001) Science 293:2108-2111; Yu, G., et al., (2000) Nat Immunol 1:252-256. The extracellular domain (ECD) of TACI contains two CRDs, the BCMA ECD comprises one CRD, and the ECD of BR3 contains only a partial CRD. Together with the receptor (Fn14) for the TWEAK ligand, BCMA and BR3 are the smallest members of the TNFR superfamily. TACI, BCMA and BR3 lack an intracellular death domain. It is believed that these receptors are involved in the survival, proliferation, and/or differentiation of a variety of cells.
The TNF family member BAFF is the only known ligand for BR3. BAFF-dependent B cell proliferation appears to require BR3; however, BAFF has also been reported to bind TACI and BCMA (Shu, H. B., and Johnson, H. (2000) Proc Natl Acad Sci USA 97:9156-9161; Thompson, J. S., et al., (2001) Science 293:2108-2111; Yu, G., et al., (2000) Nat Immunol 1:252-256). APRIL (also known as TRDL-1, TALL-2 and TNFSF13A), the TNF family member most closely related to BAFF, binds TACI and BCMA (Marsters, S. A., et al., (2000) Curr Biol 10:785-788; Thompson, J. BAFF, binds TACI and BCMA (Marsters, S. A., et al., (2000) Curr Biol 10:785-788; Thompson, J. S., et al., (2001) Science 293: 2108-2111; Yu, G., et al., (2000) Nat Immunol 1:252-256). Despite cross-reactivity with receptors, the expression patterns of BAFF and APRIL are distinct; BAFF is expressed by macrophages, monocytes, and dendritic cells, while APRIL is expressed by lymphoid cells and at elevated levels by some tumor cells (Hahne, M., et al., (1998) J Exp Med 188:1185-1190).
Tight regulation of BAFF levels appears to be critical for B cell homeostasis. BAFF knockout mice display significant reduction in the development and survival of follicular and marginal B cells while mice expressing a BAFF transgene develop a lupus-like autoimmune syndrome (Gross, J. A., et al., (2000) Nature 404:995-999; Mackay, F., et al., (2001) Science 293:2111-2114; Khare, S. D., et al., (2000) Proc Natl Acad Sci USA 97:3370-3375). A BAFF-specific antagonist, BR3-Fc (Kayagaki, N., et al., (2002) Immunity 17, 515-524), together with studies on BAFF knockout mice (Schiemann, B., et al., (2001) Science 293:2111-2114), has been used to demonstrate the essential role of BAFF in B cell development.
The role of APRIL in B cell homeostasis is less clear especially since APRIL knockout mice display normal B cell levels (E. Varfolomeev et al., (2004) Mol. Cell. Biol 24(3): 997-1006). However, several groups have reported on its activity in cell proliferation and T cell function. For example, APRIL has been shown to be capable of inducing the proliferation of certain tumors cell lines in vitro and in vivo (Hahne, M., et al., (1998) J. Exp. Med. 188:1185-1190). APRIL transgenic mice displayed augmented T cell independent B cell responses and increased survival of T cells (Stein J. et al. (2002) J Clin Invest 109:1587-98). APRIL expression has also been shown to be upregulated in many tumors including colon and prostate cancers (Hahne, M., et al., (1998) J Exp Med 188:1185-1190; Rennert, P., et al., (2000) J Exp Med 192:1677-1684; Kelly, K., et al., (2000) Cancer Res 60:1021-1027.
Both BCMA-Fe and TACI-Fc can inhibit the proliferation of primary B cells stimulated by APRIL (Yu, G., et al. (2000) Nat. Immunol. 1:252-256). Studies have shown the attenuation of autoimmune lupus-like disease progression in mice with BR3-Fc treatment (e.g., Kayagaki, N., et al., (2002) Immunity 17:515-524). A soluble form of BCMA (BCMA-Fc) has been shown to inhibit tumor cell growth in Nu/Nu mice implanted with HT29 and A549 tumor cells (Rennert, P., et al. (2000) J. Exp. Med. 192:1677-1683).
Several reports have described the nanomolar binding affinity of BAFF to BCMA (Marsters, S. A., et al., (2000) Curr Biol 10:785-788; Shu, H. B., and Johnson, H. (2000) Proc Natl Acad Sci U S A 97:9156-9161; Yu, G., et al., (2000) Nat Immunol 1:252-256), however these studies used a bivalent BCMA receptor-Fc fusion construct that could result in measured affinities that are enhanced by avidity. Pelletier et al. has anecdotally reported that a monovalent BCMA-Fc fusion protein created by mutating its Fc region interacts with BAFF with low affinity (Pelletier, M et al. (2003) JBC 278(35):33127-33133). None of these reports explored or identified the residues in BCMA that determine specificity for its binding to APRIL or BAFF as a BCMA monomer or multimer.